1. Field of the Invention
This invention relates to an automatic flash unit, and more particularly to a series control type automatic flash unit which permits high precision control of its quantity of light.
2. Description of the Prior Art
There have heretofore been known series control type and parallel control type automatic flash units. The parallel control type unit is one that a by pass tube is connected in parallel with a discharge lamp, and this type of flash unit discharges all charges stored in a main capacitor in one flashing regardless of the quantity of light output from the discharge lamp and hence does not make effective use of electrical energy; namely, the parallel control type unit has the defect that the number of times of flashing per unit time is small. In contrast thereto, the series control type unit is one that a switching element is connected in series with a discharge lamp, and in this type of flash unit, the stored charges of the main capacitor dissipated by one flashing is proportional to the quantity of light emitted by the discharge lamp and the remaining charges are used for the next flashing. Therefore, the time intervals of flashing become short, resulting in the advantage that the number of times of flashing per unit time is large if the quantity of light for each flashing is small.
FIG. 1 is an electrical circuit diagram of the conventional series control type automatic flash unit. Reference numeral 10 indicates a DC power source; 12 designates a main capacitor; 14 identifies a trigger capacitor; 16 denotes a commutation capacitor; 18 represents a light sensitive capacitor; 20 shows a trigger switch; 22 refers to a trigger transformer; 22a and 22b indicate its primary and secondary coils; 24 designates a trigger electrode; 26 identifies a discharge lamp; 30 and 42 denote silicon controlled rectifier elements (hereinafter referred to as SCR's) formig a flip-flop; 34 and 40 represent neon tubes; 36 shows a photo cell; 38, refers to an integrating capacitor; and 28, 32 and 44 to 52 indicate resistors.
In the flash unit of FIG. 1, when turning ON the trigger switch 20 ganged with a shutter of a camera after the main capacitor 12, the trigger capacitor 14, the commutation capacitor 16 and the light measuring capacitor 18 are sufficiently charged by the DC power source 10, charges stored in the trigger capacitor 14 are discharged via the primary coil 22a of the trigger transformer 22 to induce a high voltage in the secondary coil 22b, and this high voltage is applied to the trigger electrode 24, starting ionization of a rare gas sealed in the discharge lamp 26. At the same time, charges stored in the light sensitive capacitor 18 are applied across the resistor 28, and a current is applied via the resistor 32 and the neon tube 34 to the gate of the SCR 30 to conduct it, causing the discharge lamp 26 to start discharging. Simultaneously with turning ON of the trigger switch 20, a voltage by the stored charges of the light sensitive capacitor 18 is provided to a light sensitive circuit comprised of the photo cell 36 and the integrating capacitor 38, so that when the resistance value of the photoelectric conductor 36 receiving a reflected light (composed of light by flashing of the discharge lamp 26 and natural light) from a camera subject decreases with the quantity of light received, charging of the integrating capacitor 38 is started in accordance with a time constant dependent upon the resistance value of the photo cell 36 and the capacitance of the integrating capacitor 38. When the charging voltage of the integrating capacitor 38 reaches a firing voltage of the neon tube 40, the neon tube 40 is lit, applying a current to the gate of the SCR 42 to conduct it. Upon conduction of the SCR 42, the stored charges of the commutation capacitor 16 are provided to the SCR 30 to make its anode negative, so that the SCR 30 is turned OFF to cut off the current flowing in the discharge lamp 26, thus stopping it from lighting.
The resistance value of the photo cell 36 varies with the distance and the intensity of the reflected light from the subject, that is, the distance to the subject, and the intensity of the natural light, and the quantity of stored charges of the integrating capacitor 38 necessary for turning ON the neon tube 40 is predetermined to correspond to a proper exposure of a film used. Accordingly, the time from the moment of conduction of the SCR 30 to start discharging of the discharge lamp 26 to the moment of turning OFF of the SCR 30 to stop discharging of the discharge lamp 26 is controlled in accordance with the proper exposure of the film; namely, the quantity of light irradiating the subject by the discharge lamp 26 is automatically varied with the distance to the subject and the brightness thereof.
With the conventional automatic flash unit of such a construction as described above, when the SCR 42 is conducted by a light control signal from the light sensitive circuit to thereby turn OFF the SCR 30, the discharge and the re-charging current of the commutation capacitor flow through the discharge lamp 26 to cause unnecessary radiation of light, resulting in the defect of a momentary increase in the quantity of light emitted by the discharge lamp 26. This unnecessary radiation of light amounts, in terms of the quantity of light, to as large a guide number (GNO) as 3 to 5 (in the case of the film sensitivity being ASA100). In experiments of controlling the exposure, for example, to F2 through utilization of the conventional automatic flash unit, the abovesaid radiation of light caused a marked increase in the quantity of light from the discharge lamp in the cases of the distance to the subject being short, as indicated by the broken line 54 in FIG. 2 showing the ratio of variations in the quantity of light to the distance to the subject, that is, the light control characteristic; and this increase in the quantity of light was so large as not to be negligible in practical use.